High-power multi-function millimeter-wave signal generation using opo and dfg

ABSTRACT

Apparatus and method for high-power multi-function millimeter-wavelength (THz-frequency) signal generation using OPO and DFG in a single cavity. In some embodiments, the OPO-DFG cavity includes an optical parametric oscillator (OPO) non-linear material that receives pump light I P  having pump-light frequency and generates two different lower intermediate frequencies of light—an OPO-signal beam I S  and a spatially/temporally overlapping OPO-idler beam I I . A difference-frequency generator non-linear material then receives the two intermediate-frequency beams I I  and I S , and the DFG then generates a THz-frequency output signal that has a frequency equal to the difference between the two intermediate frequencies. In some embodiments, a single-piece crystal of non-linear material is used for both OPO and DFG functions. Some embodiments use a bow-tie ring having four mirrors that define the optical path: an I P -beam-entry mirror, an I P -light-extraction mirror to remove unconverted I P -beam, an I I -beam-extraction mirror, and an I S -beam-extraction mirror, and a fifth I THz -beam-extraction mirror.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 61/694,763 filed Aug. 29, 2012 byYongdan Hu et al., titled “HIGH-POWER MULTI-FUNCTION MILLIMETER-WAVESIGNAL GENERATION USING OPO AND DFG” (Attorney Docket 5032.082PV1),which is incorporated herein by reference in its entirety.

This invention is related to:

-   -   U.S. Pat. No. 7,620,077 to Angus J. Henderson, which issued on        Nov. 17, 2009, titled “Apparatus and method for pumping and        operating optical parametric oscillators using DFB fiber        lasers,” and which claimed priority to U.S. Provisional Patent        Application No. 60/697,787 filed Jul. 8, 2005,    -   U.S. Pat. No. 6,940,877 to Yongdan Hu, et al., which issued on        Sep. 6, 2005, titled “High-power narrow-linewidth        single-frequency laser,” each of which is incorporated herein by        reference in its entirety.

FIELD OF THE INVENTION

The invention relates generally to terahertz and gigahertz sources ofelectromagnetic radiation, and more particularly to apparatus andmethods for generating of two intermediate-frequency light beams from asingle laser input using an optical-parametric oscillator having a pieceof non-linear conversion material (e.g., a periodically poled lithiumniobate (PPLN) crystal or other suitable material), and then obtaining aterahertz or gigahertz signal from the two intermediate-frequency lightbeams using a non-linear optical difference-frequency generator, which,in some embodiments, uses the same piece of non-linear conversionmaterial as the OPO; or in other embodiments, uses a separate DFG pieceof non-linear conversion material in the same cavity as the piece ofnon-linear conversion material used for the OPO.

BACKGROUND OF THE INVENTION

At present, there are no millimeter-wavelength (terahertz-frequency(THz)) sources that are compact, light-weight, narrow-linewidth,tunable, and high-power sources. Referred to hereafter as THz sources,these sources output electromagnetic radiation at a frequency in therange of at least about 0.1-10 THz.

Conventional sources in the terahertz (THz) range (i.e., 0.1-10 THz)have either too-low power or are only available in limited wavelengthsand frequency bands. FIG. 1A shows the power- and frequency-capabilitycharacteristics of various conventional THz sources. 1—PCA(photoconductive antenna; e.g., interdigitated PCA); 2—OR (opticalrectification); 3—CO₂ laser frequency mixing; 4—DFG (differencefrequency generation); 5—optically pumped laser; 6—QCL (quantum-cascadelaser); 7—p-Ge-laser.

Overview of Existing Solutions—

Existing solutions have limited applications due to their correspondingsignificant issues:

-   -   PCA and OR: low power;    -   CO₂ laser frequency mixing: small spectra range/tunability and        low efficiency;    -   Traditional DFG: relatively low power, low efficiency and        complex;    -   Optically pumped laser: no tunability, gas laser—high        maintenance and bulky;    -   QCL: narrow available spectra range, requires cryogenic cooling;        and    -   p-Ge-laser: narrow available spectra range, operational        nightmare—requires a cooling Dewar, and high magnetic and pulsed        electric fields.

U.S. Pat. No. 7,054,339 issued to Yongdan Hu et al. on May 30, 2006,titled “Fiber-laser-based Terahertz sources through difference frequencygeneration (DFG) by nonlinear optical (NLO) crystals,” is incorporatedherein by reference. In the U.S. Pat. No. 7,054,339, Yongdan Hu (one ofthe inventors of the present invention) and his co-inventors described afiber-laser-based implementation of a terahertz source throughdifference frequency generation (DFG) by nonlinear optical (NLO)crystals is compact, tunable and scalable. A pair of fiber lasers(Q-switched, CW (continuous wave) or mode-locked) generatesingle-frequency outputs at frequencies ω1 and ω2. A fiber beam combinercombines the laser outputs and routes the combined output to a THzgenerator head where a nonlinear interaction process in the NLO crystalgenerates THz radiation.

U.S. Pat. No. 7,539,221 issued May 26, 2009 to Jiang, et al., titled“Fiber-laser-based gigahertz sources through difference frequencygeneration (DFG) by nonlinear optical (NLO) materials,” is incorporatedherein by reference. The U.S. Pat. No. 7,539,221 described afiber-laser-based implementation of a gigahertz source throughdifference frequency generation (DFG) by nonlinear optical (NLO)materials is compact, tunable and scalable. A pair of pulsed fiberlasers, preferably single-frequency lasers, generate output pulses atfrequencies ω1 and ω2 that overlap temporally. A beam combiner combinesthe laser outputs and routes the combined output to a GHz generator headwhere a nonlinear interaction process in the NLO material generates GHzradiation.

Optical parametric oscillators (OPOs) provide an efficient way ofconverting short-wavelength electromagnetic radiation fromcoherent-light sources to long wavelengths, while also adding thecapability to broadly tune the output wavelength. In general, an OPOsystem principally includes a short-wavelength laser source and anoptical resonator (resonant optical cavity) containing a nonlinearcrystal. In some embodiments, additional components includemode-matching optics and an optical isolator.

In general, the OPO operates with three overlapping light beams—an inputpump beam having the shortest wavelength, and thus highest frequency(typically, this is coherent light from a laser), and twolonger-wavelength, lower-frequency beams generated in the OPO called thesignal beam (this is usually called the “OPO-signal” beam herein todistinguish from other signals) and the idler beam (this is usuallycalled the “OPO-idler” beam herein). By convention, theshorter-wavelength beam is called the OPO-signal beam, and thelonger-wavelength beam is called the OPO-idler beam. The energy ofphotons in the pump beam (proportional to 1/wavelength) will equal thesum of the energy of photons in the OPO-signal beam plus the energy ofphotons in the OPO-idler beam. The pump beam (i.e., excitation lightfrom the short-wavelength laser source) is focused, using themode-matching optics, through the optical isolator and into the resonantoptical cavity, passing through the nonlinear crystal(s). Parametricfluorescence generated within the nonlinear material(s) is circulatedwithin the resonant cavity and experiences optical gain. When the OPO isexcited by a pump-power-per-unit-area above a certain threshold,oscillation occurs, and efficient conversion of pump photons toOPO-signal and OPO-idler photons occurs. Different configurations ofOPOs are possible. Variables include the wavelengths that are resonantwithin the optical cavity (pump and/or OPO-signal and/or OPO-idler) andthe type of resonator (ring versus linear). In a conventional OPO,depending on the application, either the OPO-signal beam or theOPO-idler beam, or both, will be the output light utilized by othercomponents.

U.S. Pat. No. 7,620,077 issued Nov. 17, 2009 to Angus J. Henderson (oneof the inventors of the present invention), titled “Apparatus and methodfor pumping and operating optical parametric oscillators using DFB fiberlasers,” is incorporated herein by reference. In the U.S. Pat. No.7,620,077, Henderson described an optical parametric oscillator (OPO)that efficiently converts a near-infrared laser beam to tunablemid-infrared wavelength output. In some embodiments, the OPO includes anoptical resonator containing a nonlinear crystal, such asperiodically-poled lithium niobate. The OPO is pumped by acontinuous-wave fiber-laser source having a low-power oscillator and ahigh-power amplifier, or using just a power oscillator). The fiberoscillator produces a single-frequency output defined by adistributed-feedback (DFB) structure of the fiber. The DFB-fiber-laseroutput is amplified to a pump level consistent with exceeding anoscillation threshold in the OPO in which only one of two generatedwaves (“OPO-signal” and “OPO-idler”) is resonant within the opticalcavity. This pump source provides the capability to tune the DFB fiberlaser by straining the fiber (using an attached piezoelectric element orby other means) that allows the OPO to be continuously tuned oversubstantial ranges, enabling rapid, wide continuous tuning of the OPOoutput frequency or frequencies.

U.S. Pat. No. 6,654,392 issued Nov. 25, 2003 to Arbore et al. entitled“Quasi-monolithic tunable optical resonator,” which is herebyincorporated herein by reference, describes an optical resonator havinga piezoelectric element attached to a quasi-monolithic structure thatdefines an optical path. Mirrors attached to the structure deflect lightalong the optical path. The piezoelectric element controllably strainsthe quasi-monolithic structure to change a length of the optical path byabout 1 micron. A first feedback loop coupled to the piezoelectricelement provides fine control over the cavity length. The resonator mayinclude a thermally actuated spacer attached to the cavity and a mirrorattached to the spacer. The thermally actuated spacer adjusts the cavitylength by up to about 20 microns.

A monolithic resonator typically includes a single block of transparentmaterial having reflecting facets that serve as the mirrors. Usually,the material is strained by changing its temperature. U.S. Pat. No.4,829,532 issued May 9, 1989 to Kane, which is hereby incorporatedherein by reference, describes an alternative where the optical pathlength of a monolithic oscillator can be adjusted by a piezoelectricelement mounted to uniformly strain the entire block in a plane parallelto the plane of the optical path.

U.S. Pat. No. 8,035,083 issued Oct. 11, 2011 to Kozlov et al., titled“Terahertz tunable sources, spectrometers, and imaging systems,” isincorporated herein by reference. Kozlov et al. describe a source ofterahertz radiation at a fundamental terahertz frequency that is tunableover a fundamental terahertz-frequency range, and is coupled into afirst waveguide. The first waveguide supports only a single transversespatial mode within the fundamental terahertz frequency range. Asolid-state frequency multiplier receives from the first waveguide theterahertz radiation and produces terahertz radiation at a harmonicterahertz frequency. A second waveguide receives the harmonic terahertzradiation. The tunable terahertz source can include a backward-waveoscillator with output tunable over about 0.10-0.18 THz, 0.18-0.26 THz,or 0.2-0.37 THz. The frequency multiplier can include at least onevaristor or Schottky diode, and can include a doubler, tripler, pair ofdoublers, doubler and tripler, or pair of triplers. The terahertz sourcecan be incorporated into a terahertz spectrometer or a terahertz imagingsystem.

U.S. Pat. No. 7,421,171 issued Sep. 2, 2008 to Ibanescu et al., titled“Efficient terahertz sources by optical rectification in photoniccrystals and meta-materials exploiting tailored transverse dispersionrelations,” is incorporated herein by reference. Ibanescu et al.describe generating terahertz (THz) radiation. Their system includes aphotonic-crystal structure including at least one nonlinear materialthat enables optical rectification. The photonic-crystal structure isconfigured to have the suitable transverse dispersion relations andenhanced density photonic states so as to allow THz radiation to beemitted efficiently when an optical or near-infrared pulse travelsthrough the nonlinear part of the photonic crystal.

U.S. Pat. No. 7,473,898 issued Jan. 6, 2009 to Holly et al., titled“Cryogenic terahertz spectroscopy,” is incorporated herein by reference.Holly et al. describe a terahertz spectroscopy system that includes asource of terahertz radiation, a detector of terahertz radiation, asource of sample gas, and a sample cell that can be cooled to cryogenictemperatures. The sample cell may be configured to receive the samplegas, received terahertz radiation from the source of terahertzradiation, provide the terahertz radiation to the detector after theterahertz radiation has passed through the sample gas, and facilitatecryogenic cooling thereof. The sample cell may be cryogenically cooledto freeze the sample gas and subsequently warmed either continuously orin steps in temperature so that individual components or groups ofcomponents of the sample gas may evaporate and thus have absorptionspectra formed therefor.

U.S. Patent Application Publication US 2005/0018298 of Trotz et al.,published Jan. 27, 2005 and titled “Method and apparatus for generatingterahertz radiation,” is incorporated herein by reference. Trotz et al.describe generating terahertz radiation. Their terahertz source isdescribed as a versatile terahertz device that can be configured totransmit a plurality of wavelengths, thereby facilitating the detectionof multiple contaminants using a single source device. In oneembodiment, the Smith-Purcell radiation effect is exploited by passingan electron beam over a modulated conducting surface, wherein thespacing of the periods of the modulated surface is varied. Thevariations in the modulated surface enable the source to produce lightof varying wavelengths.

U.S. Pat. No. 7,781,737 issued Aug. 24, 2010 to Zhdaneev, titled“Apparatus and methods for oil-water-gas analysis using terahertzradiation,” is incorporated herein by reference. Zhdaneev describesanalyzing gas-oil-water compounds in oilfield and other applicationsusing terahertz radiation. A sample analyzer includes a sample chamberhaving a fluid communication port configured to receive the sample. Theanalyzer also includes a filter to filter samples and selectively removeoil, water or gas from reservoir mixture received by the sample chamber.A terahertz (THz) radiation detector is provided in electromagneticcommunication with the sample. The terahertz detector provides adetected output signal indicative of the terahertz electromagneticradiation detected from the sample. In some embodiments, the device alsoincludes a terahertz source illuminating the sample, the terahertzdetector detecting a portion of the terahertz source illumination asmodified by the sample. The detected portion of the spectrum ofterahertz radiation can be processed to analyze the composition of thesample.

U.S. Pat. No. 7,995,628 issued Aug. 9, 2011 to Wu, titled “Recyclingpump-beam method and system for a high-power terahertz parametricsource,” is incorporated herein by reference. Wu describes thefabrication of a portable high-power terahertz beam source that canproduce what Wu calls a tunable, high-power terahertz beam over thefrequency from 0.1 THz to 2.5 THz. Wu's terahertz source employs arecycling pump beam method and a beam quality-control device. The beamquality-control device may or may not be required for a high-powerterahertz beam generation. In exemplary embodiments, a lithium niobate(LiNbO₃) crystal or a lithium niobate crystal doped with 5% magnesiumoxide (LiNbO₃:MgO) can be used. Other nonlinear optical crystals,including GaSe can be used in place of the LiNbO₃ crystal. Throughproper alignment of a pump beam, along with recycling a pump beam, highconversion efficiency is achieved, and a high output power beam isproduced at terahertz frequencies.

U.S. Pat. No. 7,391,561 (Attorney Docket 5032.008US1) titled “Fiber- orrod-based optical source featuring a large-core, rare-earth-dopedphotonic-crystal device for generation of high-power pulsed radiationand method,” which issued to Di Teodoro, et al. on Jun. 24, 2008, isincorporated herein by reference. Di Teodoro, et al. describe aphotonic-crystal fiber having a very large core while maintaining asingle transverse mode. The typical problems of multiple-modes and modehopping, which result from the use of large-diameter waveguides, areaddressed by the invention. By using multiple small waveguides inparallel, large amounts of energy can be passed through a laser, butwith better control such that the aforementioned problems can bereduced. An additional advantage is that the polarization of the lightcan be better maintained as compared to using a single fiber core.

There is still a heretofore unmet need in the art for an improved methodand apparatus for high-power fiber-laser-based gigahertz-to-terahertz,millimeter-wave, signal sources for advanced sensors, photonics, andoptical computing.

BRIEF SUMMARY OF THE INVENTION

Terahertz technology is a very promising technology for both defense andcommercial applications. In some embodiments, the present inventionprovides a high-power fiber-laser-based gigahertz-to-terahertz,millimeter-wave, signal-generation apparatus that provides key wantedfeatures in a compact and light-weight package. In some embodiments, thepresent invention offers at least ten times the output power as previousconventional sources, while having multiple functions incorporated inone system.

In some embodiments, the present invention uses a high-power infrared(IR) laser (e.g., an IR fiber laser provides an advantageous solutionsin some environments) as a source of pump-laser energy to pump a tunableoptical parametric oscillator (OPO) crystal plus adifference-frequency-generation (DFG) crystal to generatenarrow-linewidth, tunable and high-power signals in the GHz to THz rangea compact and light-weight package while having an option to useresidual high-power IR laser beam (i.e., the pump, OPO-idler and/orOPO-signal wavelengths) for other useful applications in the sameinstrument. In some embodiments, a single crystal is used for both theOPO and the DFG. The OPO function is used to receive a pump frequencyand use that energy to generate an OPO-idler frequency and an OPO-signalfrequency that differ from one another by the desired THz or GHzfrequency that is to be output. For example, in some embodiments, aninfrared (IR) pump wavelength of 1060 nm has a frequency of 283.0188679THz, and is used to generate OPO-signal and OPO-idler wavelengths of,e.g., about 2112.5357072 nm and about 2127.517228 nm (corresponding toOPO-signal and OPO-idler frequencies of about 142.009434 THz and about141.009434 THz), which are then used as electromagnetic-radiation inputsto a DFG that outputs the frequency difference of about 1.00000002 THz.

In some embodiments, the OPO and the DFG are located in the same cavity,in some embodiments, the OPO and the DFG are implemented using the sametype of non-linear optical crystal, and in some embodiments, the OPO andthe DFG are implemented using the same non-linear optical crystal.

Some embodiments provide an apparatus and a related method forhigh-power multi-function millimeter-wavelength (THz-frequency) signalgeneration using an OPO and a DFG in a single cavity. In someembodiments, the OPO-DFG cavity includes an optical parametricoscillator (OPO) non-linear material that receives pump light I_(P)having pump-light frequency and generates two lower intermediatefrequencies of light—an OPO-signal beam I_(S) and a spatiallyoverlapping OPO-idler beam I_(I). A difference-frequency generatornon-linear material then receives the two intermediate-frequency beamsI_(I) and I_(S), and the DFG then generates a THz-frequency outputsignal that has a frequency equal to the difference between the twointermediate frequencies. In some embodiments, a single-piece crystal ofnon-linear material is used for both OPO and DFG functions. Someembodiments use a bow-tie ring having four mirrors that define fourcorners of the bow-tie-shaped optical path: a frequency-selectiveI_(P)-beam-entry mirror (in some embodiments, this first mirror ishighly transmissive to I_(P) and highly reflective to I_(S) and/orI_(I)), a frequency-selective I_(P)-light-extraction mirror (in someembodiments, this second mirror is highly transmissive to I_(P) andhighly reflective to I_(S) and/or I_(I)) to remove unconvertedI_(P)-beam, a frequency-selective or partially transmissiveI_(I)-beam-extraction mirror (in some embodiments, this third mirror ispartially transmissive to I_(I) and highly reflective to I_(S)), and afrequency-selective or partially transmissive I_(S)-beam-extractionmirror (in some embodiments, this fourth mirror is partiallytransmissive and partially reflective to I_(S)), and optionally a fifthI_(THz)-beam-extraction mirror within the bow-tie optical path.Alternatively, the I_(P)-light-extraction mirror is highly transmissiveto both I_(THz) and I_(P), and is used to extract both the unconvertedI_(P) light and the I_(THz) output beam, which are then separated by abeam-separation mirror external to the bow-tie path. Other embodimentsuse two diffraction gratings to split and recombine the two intermediatefrequencies (or to select one of the intermediate-frequency beams(either the OPO-signal beam I_(S) or the OPO-idler beam I_(I)) as thefixed-intermediate-frequency circulating beam and to dump the other oneof the intermediate-frequency beams, and two corner mirrors, whichtogether define the bow-tie optical path.

Other embodiments use two diffraction gratings, one used to split thetwo intermediate frequencies into separate beams, and the other gratingused to recombine the two intermediate-frequency beams, and two (ormore, if separate mirrors are used for the two beams) corner mirrors. Insome such embodiments, one or more etalons are used to frequency-filtereach of the two intermediate-frequency beams independently to theirrespective frequencies, and two cylindrical mirrors are used tore-converge each of the two separated beams toward the seconddiffraction grating. In some such embodiments, a differentpiezo-electric actuator is attached to each of the two mirrors that areused to re-converge each of the two separated beams toward the seconddiffraction grating, wherein the two piezo-electric actuators allowindependent adjustment of the lengths of the cavity as seen by eachintermediate beam, for tuning purposes.

One advantage of this solution is that it provides a light-weight systemthat outputs a tunable, high-power, THz and/or GHz signal that, in someembodiments, has a narrow linewidth.

BRIEF DESCRIPTION OF THE FIGURES

Each of the items shown in the following brief description of thedrawings represents some embodiments of the present invention.

FIG. 1A is a graphical representation of the power ranges (verticalaxis) and frequency ranges (horizontal axis) available from each of aplurality of conventional THz sources.

FIG. 1B is a graphical representation of the power range (vertical axis)and frequency range (horizontal axis) available from embodiments of thepresent invention, as compared to each of a plurality of conventionalTHz sources as shown in FIG. 1A.

FIG. 1C is a block diagram of a distributed-feedback fiber laser 100having a tuning mechanism.

FIG. 2 is a block diagram of a system 200 having a two-mirror combinedOPO and DFG pumped by a fiber laser.

FIG. 3A is a block diagram of an alternative four-mirror bow-tiecombined OPO and DFG cavity device 301.

FIG. 3B is a block diagram of another four-mirror bow-tie combined OPOand DFG cavity device 302.

FIG. 4A is a block diagram of another four-mirror bow-tie combined OPOand DFG cavity device 401.

FIG. 4B is a block diagram of another four-mirror bow-tie combined OPOand DFG cavity device 402.

FIG. 5A is an elevation-view block diagram of another four-mirrorbow-tie combined OPO and DFG cavity device 501.

FIG. 5B is a plan-view block diagram of the four-mirror bow-tie combinedOPO and DFG cavity device 501 shown in FIG. 5A.

DETAILED DESCRIPTION OF THE INVENTION

Although the following detailed description contains many specifics forthe purpose of illustration, a person of ordinary skill in the art willappreciate that many variations and alterations to the following detailsare within the scope of the invention. Very narrow and specific examplesare used to illustrate particular embodiments; however, the inventiondescribed in the claims is not intended to be limited to only theseexamples, but rather includes the full scope of the attached claims.Accordingly, the following preferred embodiments of the invention areset forth without any loss of generality to, and without imposinglimitations upon the claimed invention. Further, in the followingdetailed description of the preferred embodiments, reference is made tothe accompanying drawings that form a part hereof, and in which areshown by way of illustration specific embodiments in which the inventionmay be practiced. It is understood that other embodiments may beutilized and structural changes may be made without departing from thescope of the present invention.

The embodiments shown in the Figures and described here may includefeatures that are not included in all specific embodiments. A particularembodiment may include only a subset of all of the features described,or a particular embodiment may include all of the features described.

The leading digit(s) of reference numbers appearing in the Figuresgenerally corresponds to the Figure number in which that component isfirst introduced, such that the same reference number is used throughoutto refer to an identical component which appears in multiple Figures.Signals and connections may be referred to by the same reference numberor label, and the actual meaning will be clear from its use in thecontext of the description.

FIG. 1A is a graphical representation of the approximate power ranges(vertical axis) and frequency ranges (horizontal axis) available fromeach of a plurality of conventional THz sources. Conventional OR THzsources (with power/frequency ranges of oval 111) appear to have a powerrange of approximately 5×10⁻⁸ watts (5×10⁻⁵ mW) to approximately 1×10⁻⁵watts (1×10⁻² mW), and a frequency range of approximately 100 GHz (0.1THz) to approximately 30 THz. Conventional PCA THz sources (withpower/frequency ranges of oval 112) appear to have a power range ofapproximately 5×10⁻⁸ watts (5×10⁻⁵ mW) to approximately 2×10⁻⁴ watts(2×10⁻¹ mW), and a frequency range of approximately 100 GHz (0.1 THz) toapproximately 5 THz. Conventional CO₂-based laser-frequency-mixing THzsources (with power/frequency ranges of oval 113) appear to have a powerrange of approximately 5×10⁻⁶ watts (5×10⁻³ mW) to approximately 5×10⁻⁴watts (2×10⁻¹ mW) and a frequency range of approximately 100 GHz (0.1THz) to approximately 50 THz, and a frequency range of approximately 200GHz (0.2 THz) to approximately 5 THz. Conventional traditional DFG THzsources (with power/frequency ranges of oval 114) appear to have a powerrange of approximately 1×10⁻⁴ watts (0.1 mW) to approximately 0.1 watts(100 mW), and a frequency range of approximately 200 GHz (0.2 THz) toapproximately 5 THz. Conventional optically pumped laser THz sources(with power/frequency ranges of oval 115) appear to have a power rangeof approximately 5×10⁻² watts (50 mW) to approximately 0.5 watts (500mW), and a frequency range of approximately 100 GHz (0.1 THz) toapproximately 5 THz. Conventional quantum-cascade-laser THz sources(with power/frequency ranges of oval 116) appear to have a power rangeof approximately 0.5 watts (500 mW) to approximately 5 watts (5,000 mW),and a frequency range of approximately 800 GHz (0.8 THz) toapproximately 2 THz. Conventional p-Ge-laser THz sources (withpower/frequency ranges of oval 117) appear to have a power range ofapproximately 5 watts to approximately 100 W, and a frequency range ofapproximately 800 GHz (0.8 THz) to approximately 2 THz.

FIG. 1B is a graphical representation of the power range (vertical axis)and frequency range (horizontal axis) available from embodiments of thepresent invention, as compared to each of a plurality of conventionalTHz sources as shown in FIG. 1A. In some embodiments, the OPO-DFG THzsources (with power/frequency ranges of oval 118) typically have a powerrange of approximately 0.01 watts to 100 W or more, and a frequencyrange of 30 GHz (0.03 THz) or less, to 50 THz or more.

In some embodiments, the present invention uses a high-power infrared(IR) laser—in some embodiments, a fiber laser—to pump a unit includingtunable OPO plus DFG (difference-frequency-generation) crystals togenerate narrow-linewidth, tunable and high-power THz sources in acompact and light-weight package, while having an option to use residualhigh-power IR laser beam for other useful applications.

FIG. 1C is a block diagram of a distributed-feedback fiber laser 100having a tuning mechanism, which, in some embodiments, is used as thepump source or as the master-oscillator seed pump signal to a poweramplified (MOPA) pump-light source for the present invention. In someembodiments, the present invention provides anoptical-parametric-oscillator difference-frequency-generator (OPO-DFG)THz source having continuous tuning that is enabled by using adistributed-feedback fiber laser 100 such as shown in FIG. 1C (anddescribed in U.S. Pat. No. 7,620,077 titled “Apparatus and method forpumping and operating optical parametric oscillators using DFB fiberlasers,” which is incorporated herein by reference in its entirety) asits pump source. One embodiment of such a distributed-feedback fiberlaser 100 includes an optical fiber 120 having a core 122 and one ormore cladding layers 121. In some embodiments, a portion of fiber 120includes a distributed-feedback grating 123 having a gap 124 that has alength δl_(GAP)=λ_(B)/4, or λ_(B)(N+0.25) where N is an integer. In someembodiments, a tuning mechanism (such as, for example, a PZT(piezo-electric element) and/or a heating element used to stretch thelength of grating 123 and/or gap 124) is used to change the wavelengthof the laser light output of DFB fiber laser pump source 100. In someembodiments, DFB-pump light 125 of a suitable wavelength (e.g., in someembodiments, 980 nm wavelength is used for the optical excitation 125 topump the DFB laser 100) is input to the DFB laser, and laser light 126and 127 is output having a wavelength useful for pumping any of theOPO-DFG devices described below. In some embodiments, pump laser 100 isused for master oscillator 211 described below.

FIG. 2 is a block diagram of a system 200 having a two-mirror combinedOPO and DFG pumped by a fiber laser. In some embodiments, system 200includes a pump laser 210 and an OPO-DFG 220. In some embodiments, pumplaser 210 includes a master oscillator 211 (e.g., a DFB tunable fiberlaser such as described above in FIG. 1C, in some embodiments) and anoptical power amplifier 212. In other embodiments, only a power masteroscillator 211 (e.g., a powerful DFB tunable fiber laser) is used, whenthat provides sufficient laser pump light power by itself. In someembodiments, fiber connector 213 connects pump laser 210 to OPO-DFG 220.In some embodiments, OPO-DFG 220 includes a first collimating lens 221,a Faraday isolator 222, and a second collimating lens 223 that togethercondition the pump light for injection into OPO-DFG resonator 230,which, in some embodiments, is tuned (e.g., by PZT 227 moving mirror 226to adjust the length of the OPO-DFG cavity between input mirror 224 andoutput mirror 226) to resonate at the OPO-signal wavelength (and/or (insome embodiments) at the OPO-idler wavelength and/or (in someembodiments) at the pump wavelength). In some embodiments, theOPO-signal frequency is kept at a fixed frequency, such that as the pumpwavelength is changed, the OPO-idler frequency is also changed in orderto tune the frequency of the THz output signal, such that the DFGnon-linear crystal generates a THz signal having a frequency equal tothe difference between the OPO-signal frequency and the OPO-idlerfrequency. In some embodiments, a non-linear crystal 225 (such as, e.g.,periodically poled lithium niobate (PPLN)) is used to convert thepump-wavelength light (having the shortest wavelength) intoOPO-signal-wavelength light (having a wavelength between that of thepump wavelength and that of the OPO-idler wavelength) andOPO-idler-wavelength light (having the longest wavelength). In someembodiments, M2 mirror 227 is wavelength-selective such that it passesthe THz output signal and perhaps some pump, OPO-signal and/or OPO-idlerlight. In some embodiments, lens 229 collimates, andwavelength-selective mirror 231 removes, the pump light (directeddownward) and they pass THz output signal 250 as output to the right.The OPO-DFG resonator cavity 230 is generally desired to resonate atonly a single wavelength (e.g., the OPO-signal wavelength ifOPO-idler-wavelength light is the tunable variable-wavelength input tothe DFG 226 (for example, the longer wavelength is the tunablewavelength that is input to the DFG 226), or the OPO-idler wavelength ifOPO-signal-wavelength light is the variable-wavelength input to the DFG226 (for example, the shorter wavelength is the tunable wavelength thatis input to the DFG 226)). In some embodiments, mirror 227 (which, insome embodiments, is adjusted using piezo element 228 to tune the fixedresonant wavelength (either OPO-signal or OPO-idler wavelength)) is madesubstantially transparent or non-reflective at the pump wavelength andat the wavelength of the variable-wavelength input to the DFG 226, inorder that those wavelengths do not become resonant in the cavity (insome embodiments, if two or more wavelengths (of the pump, OPO-signal,and OPO-idler wavelengths) become resonant, then wavelength and/oramplitude instabilities ensue since power may shift between the resonantwavelengths). To avoid such instabilities, some (or most) embodiments ofthe present invention are designed to resonate at a single wavelength(of the pump, OPO-signal, and OPO-idler wavelengths), and some or all ofthe mirrors are designed to transmit other wavelengths that may arise.

FIG. 3A is a block diagram of an alternative four-mirror bow-tiecombined OPO and DFG cavity device 301. In some embodiments, OPO-DFG 320accepts input pump light from fiber 310 (in some embodiments, comingfrom a tunable DFB fiber laser, not shown), and includes afiber-pigtail-input Faraday isolator 322, and a collimating lens 323that together condition the pump light for injection into OPO-DFGresonator 330, which, in some embodiments, is tuned (e.g., by PZT 327moving mirror 331 to adjust the length of the OPO cavity between inputmirror 324 and itself (i.e., the path that returns to mirror 324) by wayof output mirror 326, movable mirror 331, and mirror 332) to resonate atthe OPO-signal wavelength (and/or the OPO-idler wavelength and/or thepump wavelength). In some embodiments, moving mirror 331 is highlyreflective at the frequency of OPO-signal I_(S), and transmissive at thefrequency of OPO-idler I_(I), such that OPO-idler I_(I) leaves thebow-tie ring 391 as I_(I) output beam 351. In some embodiments, anetalon 333 that passes just the OPO-signal wavelength is also used (inaddition to movable mirror 331), or alternatively used, to tune theresonant OPO-signal wavelength λ_(S) to match the path length of bow-tiering 391. In some embodiments, mirror 332 is partially reflective andpartially transmissive at the frequency of OPO-signal I_(S), such thatsome of OPO-signal I_(S) leaves the bow-tie ring 391 as I_(S) outputbeam 352. In some embodiments, the OPO-signal frequency (of light I) iskept at a fixed frequency, such that as the pump frequency of pump lightI_(P) is changed, the OPO-idler frequency (of light I_(I)) is alsochanged, and thus the THz output-signal frequency is changed by the sameamount. In other embodiments, a fixed-frequency pump light I_(P) isused, a fixed-frequency light I_(I) is used, a fixed-frequency lightI_(S) is used, and a fixed-frequency THz output signal is generated.

In some embodiments, a non-linear crystal 325 (such as, e.g., PPLN) isused to convert the pump-wavelength light I_(P) (the photons having thehighest frequency) into OPO-signal-wavelength light I_(S) (photonshaving a frequency between the pump frequency and the OPO-idlerfrequency, which enter the DFG crystal 335) and OPO-idler-wavelengthlight I_(I) (photons having the lowest intermediate frequency enteringthe DFG crystal 335). In some embodiments, non-linear crystal 325 and/ornon-linear crystal 335 are/is heated and kept at a constant temperaturein an oven. In some embodiments, non-linear crystal 325 is touching (asshown in FIG. 3A) or closely adjacent to non-linear crystal 335, whilein other embodiments (not shown here), there is a spatial separationbetween non-linear crystal 325 and non-linear crystal 335. In some otherembodiments, the cavity 330 is made to be resonant at the OPO-idlerfrequency, and in this case, the OPO-signal frequency (i.e., a frequencyhigher than the OPO-idler frequency) is the variable-frequency and/ornon-resonant light entering DFG 335 to produce the THz output signalelectromagnetic radiation 350. In some embodiments, M2 mirror 326 iswavelength-selective such that it reflects some or all of the resonantand/or non-resonant intermediate-frequency light (i.e., the OPO-idlerlight I_(I) and/or OPO-signal light I_(S), as the case may be, isreflected (directed leftward and slightly downward)) and but passes theunconverted pump light and the THz output signal I_(THz). In someembodiments, wavelength-selective mirror 353 passes the pump light I_(P)329 and any leaked OPO-idler light I_(I) and/or OPO-signal light I_(S)(directed rightward), but reflects THz output-signal electromagneticradiation I_(THz) 350 as output (upward in FIG. 3A). In some otherembodiments (not shown here), wavelength-selective mirror 353 reflectsthe pump light I_(P), and any leaked OPO-idler light I_(I) and/orOPO-signal light I_(S) (directed upward) and passes THz output-signalelectromagnetic radiation I_(THz) 350 as output (which would berightward if depicted in FIG. 3A).

In some embodiments, non-linear crystal 325 and/or non-linear crystal335 have/has periodically alternating ferroelectric domain structuresthat vary in period (the poling period) across the width of the crystal(such crystals are called periodically poled; e.g., PPLN is periodicallypoled lithium niobate). In some embodiments, the sideways positioning ofnon-linear crystal 325 and/or non-linear crystal 335 can be varied, inorder to vary the poling period encountered by the light propagatingthrough the crystal(s). The OPO-DFG resonator 320 includes the bow-tiering path 391 (i.e., the bow-tie ring 391 being the optical path frommirror 324, through OPO 325 and DFG 335 to mirror 326, then to mirror331, then through etalon 333 to mirror 332, and finally back to mirror324), configured to avoid unwanted double or triple resonances bycirculating only one of the two intermediate-frequency beams (e.g.,OPO-signal light I) and to maintain a sufficient amount of pump lightI_(P) and a sufficient amount of OPO-signal light I_(S) such that OPO325 is above threshold in order to generate the OPO-idler light I_(I),such that amounts of the OPO-signal light I_(S) and OPO-idler lightI_(I) needed by DFG 335 to generate the THz output light are maintained.When OPO-signal light I_(S) is the frequency that circulates aroundbow-tie ring path 391, the amount of OPO-signal light I_(S) exceeds theamount of OPO-idler light I_(I), since one photon of OPO-pump lightI_(P) and one photon of OPO-signal light I_(S) will cause the loss ofthe one OPO-pump light I_(P) photon and the emission of one photon ofOPO-idler light I_(I) and one photon of OPO-signal light I_(S) inaddition to the starting one photon of OPO-signal light I_(S). In otherembodiments, it is the OPO-idler light I_(I) (the lower-frequency of thetwo intermediate beams) that is circulated around bow-tie ring path 391(including through etalon 333), and the OPO-signal light I_(S) isremoved by mirror 331 and/or etalon 333.

FIG. 3B is a block diagram of another four-mirror bow-tie combined OPOand DFG cavity device 302 having certain parts corresponding to those ofFIG. 3A, and showing a MOPA with master-oscillator (the DFB fiber laser305) and power-amplifier (the fiber amplifier 308) shown. In contrast tothe device 301 embodiment shown in FIG. 3A, device 302 separates OPOcrystal 325 from DFG crystal 335 so that each is in a separaterespective leg of the bow-tie ring 392 (i.e., bow-tie ring 392 being theoptical path from mirror 324, through OPO 325 to mirror 346, thenthrough DFG 335 and mirror 353 to mirror 331, then through etalon 333 tomirror 332, and finally back to mirror 324), which allows removal of thepump light I_(P) through wavelength-selective mirror 346 before theOPO-idler light I_(I) and OPO-signal light I_(S) enter the DFG crystal335 to be converted to electromagnetic radiation of the THzoutput-signal frequency.

In some embodiments, fiber laser 309 (e.g., in some embodiments, seededby a DFB fiber laser such as shown in FIG. 1C; or in other embodiments,seeded by a Q-switched fiber laser 305) is optically pumped from one ormore laser diodes, is configured in a MOPA arrangement, which optionallyuses one or more high-power photonic-crystal amplifiers as fiberamplifier 308 (e.g., shown here as a loop of fiber, but alternativelyusing a laser and/or power amplifier such as those described incommonly-assigned U.S. Pat. No. 7,391,561 titled “Fiber- or rod-basedoptical source featuring a large-core, rare-earth-doped photonic-crystaldevice for generation of high-power pulsed radiation and method,” whichis incorporated herein by reference). In some embodiments, laser 309includes a fiber isolator 306, fiber connector 307 (to fiber amplifier308) and output fiber 310 (the diode pump laser(s), not shown, is/arecoupled using methods well known to the art, such as shown in U.S. Pat.No. 7,391,561).

In some embodiments, the output beam of pump laser 309 is collimatedusing lens 341, reflected by mirror 342, passed through one-way bulkisolator 322, and reflected by mirror 343 through focusing lens 323 thatacts to focus (taking into account the focusing effects of mirror 324)the pump beam at the center of the PPLN 325. Pump-extraction mirror 346is transparent to the pump frequency (which is output through mirror326) (in order to prevent a doubly or triply resonant cavity), whilebeing highly reflective to light at the OPO-signal frequency and theOPO-idler frequency. The coincident beams I_(S) and I_(I) containing theintermediate OPO-signal frequency and the OPO-idler frequency are usedby DFG 335 to generate the THz output that is reflected offfrequency-selective mirror 353 to become THz output 350. In someembodiments, frequency-selective mirror 353 passes the unconvertedportions of intermediate OPO-signal I_(S) and OPO-idler I_(I). In someembodiments, the mirror 331 is highly reflective at the resonantfrequency (e.g., the OPO-signal frequency), and highly (or at leastpartially) transmissive at the non-resonant intermediate (e.g.,OPO-idler) frequency, the etalon 333 is tuned to be transparent only atthe resonant (e.g., OPO-signal) frequency (blocking or reflecting any ofthe other intermediate frequency that may have been reflected by mirror331), and mirror 332 is highly reflective at the resonant (e.g.,OPO-signal) frequency, such that only that frequency returns to inputmirror 324. In some other embodiments, the mirror 331 need not betransmissive at the non-resonant intermediate (e.g., OPO-idler)frequency since that frequency should be blocked by etalon 333.

In some embodiments, as described further below, one or more of themirrors 324, 326, 331, and/or 332 is partially transparent to theOPO-signal frequency and/or OPO-idler frequency (e.g., one- tofive-percent transparent in total), in order to prevent excessivebuildup of light at that/those frequency(ies) in the cavity, which wouldtend to overheat the crystal(s) 325 and/or 335. In some embodiments (asshown in FIG. 3B), the THz output signal 350 is extracted by reflectionat frequency-selective mirror 353 within the bow-tie ring. In some otherembodiments (not shown), mirror 353 is omitted and the THz output signal350 is extracted by transmission through a THz-transmissive,I_(S)-reflective and/or I_(I)-reflective, frequency-selective mirrorthat replaces mirror 331. Thus, in some embodiments, the unconvertedpump light I_(P) is removed from the bow-tie ring of resonator 330 onceit leaves OPO 325 and before the remaining OPO-signal light I_(S) andOPO-idler light I_(I) enter the DFG 335 (e.g., in the embodiment shown,unconverted pump light I_(P) passes through frequency-selective mirror346, while the remaining OPO-signal light I_(S) and OPO-idler lightI_(I) are reflected toward DFG 335). Further, in some embodiments, theTHz output electromagnetic radiation I_(Txz) is removed from the bow-tiering of resonator 330 once it leaves DFG 335 and before the remainingOPO-signal light I_(S) and OPO-idler light I_(I) impinge on mirror 331and/or etalon 333, which allows one of those to pass (e.g., in someembodiments, remaining OPO-signal light I_(S)), while the otherOPO-idler light I_(I) is blocked and/or removed from the bow-tie ring).

FIG. 4A is a block diagram of another four-mirror bow-tie combined OPOand DFG cavity device 401. In some embodiments, pump source 411 suppliespump light I_(P) that passes through frequency-selective mirror 324 thatis highly transmissive to the frequency of pump light I_(P) and highlyreflective to the frequency of at least one of theintermediate-frequency beams of light I_(S) and/or I_(I). In someembodiments, the pump light I_(P) and OPO-signal light I_(S) enter OPOcrystal 325, and stimulate absorption of pump light I_(P) and emissionof OPO-idler light I_(I) and additional OPO-signal light I_(S) (denotedas I_(P)+I_(I)+I_(S) propagating rightward in the FIG. 4A). In someembodiments, pump-extraction mirror 346 is highly transmissive to thefrequency of pump light I_(P), and highly reflective to the frequency ofOPO-idler light I_(I) and to the frequency OPO-signal light I_(S)(denoted as I_(I)+I_(S) propagating leftward and downward in FIG. 4A),that enter DFG crystal 335, which generates the difference-frequencysignal I_(THz) that is output, by reflection by frequency-selectivemirror 353 (which transmits the frequency of OPO-idler light I_(I) andto the frequency OPO-signal light I_(S) but reflects THz signals), asTHz output signal 350. The unconverted OPO-idler light I_(I) andOPO-signal light I_(S) are transmitted through frequency-selectivemirror 353 and are diffracted back (upward and rightward) bybeam-separating diffraction grating 412 (in some embodiments, the beamof unconverted OPO-idler light I_(I) and OPO-signal light I_(S) impingeson beam-separating diffraction grating 412 at or very near to theLittrow angle (wherein the outgoing OPO-idler light beam I_(I) willdiffract back to a small angle to one side of the incoming beam tograting 412, and the outgoing OPO-signal light beam I_(S) will diffractback to a small angle to the opposite side of the incoming beam tograting 412) in order to obtain the most light in the two primarydiffracted beams OPO-idler light I_(I) and OPO-signal light I_(S), whichdiffract back toward mirror 431 at slightly different angles due totheir slightly different frequencies. In some embodiments, mirror 431 isagain mounted to a piezo-electric (PZT) actuator 327 used to tune theresonant cavity length, but unlike mirror 331 of FIG. 3A, in someembodiments, mirror 431 is highly reflective to both I_(I) and I_(S). Insome embodiments, an additional PZT actuator (not shown here) is coupledto one or the other of mirrors 448 or 449, so that PZT actuator 327 isused to tune the length of cavity for the intermediate-frequency beamthat reflects from the fixed one of mirrors 448 or 449, and then theadditional PZT actuator is used to move the other one of mirrors 448 or449 to tune the length of the cavity seen by the otherintermediate-frequency beam. Because of the angular separation (which iscaused by beam-separating diffraction grating 412) between OPO-idlerlight I_(I) and OPO-signal light I_(S), separate etalons 439 and 438 areused to individually and independently tune the respective frequenciesof OPO-idler light I_(I) and OPO-signal light I_(S) to obtain thedesired output frequency of THz output signal 350. In some embodiments,cylindrical mirrors 449 and 448, respectively, are used to reverse thedivergence of the separate beams of OPO-idler light I_(I) and OPO-signallight I_(S) (which divergence is caused by slightly different angles ofdiffraction for frequencies at opposite edges of the linewidth of eachbeam) in the plane of ring 393 and have them converge to beam-combiningdiffraction grating 413, which, in some embodiments, is also at anear-Littrow angle (wherein the incoming OPO-idler light beam I_(I) willimpinge at a small angle to one side of the outgoing beam from grating413, and the incoming OPO-signal light beam I_(S) will impinge at asmall angle to the opposite side of the outgoing beam to grating 412,and both frequencies will thus recombine into a single collimated beam)to obtain maximum efficiency in diffracting and recombining the twoseparate beams of OPO-idler light I_(I) and OPO-signal light I_(S) intoa single beam directed at mirror 324. In some embodiments,beam-combining diffraction grating 413 (which, by itself, cannotcompensate for the spatial-divergence widening within each beam, andthus would produce a wider beam propagating toward OPO 325 than desired)and cylindrical mirrors 448 and 449 (which, by themselves, cannotcompensate for the angular difference between the two beams, and thuscould not produce a single collimated beam as desired) together providethe requisite compensation to both reverse the chromatic dispersion ordivergence within each beam and the angular separation between OPO-idlerlight I_(I) and OPO-signal light I_(S), both of which were introduced bybeam-separating diffraction grating 412 earlier in the ring.

In some such embodiments, one etalon is used to frequency-filter both ofthe two intermediate-frequency beams (because the two beams traverse theetalon at different angles, a single etalon (such as 537 of FIG. 5A) canbe used to tune both of two different frequencies) or in otherembodiments, one or more etalons 438 and 439 are used tofrequency-filter each of the two intermediate-frequency beamsindependently to their respective frequencies, for tuning purposes. Insome embodiments, two cylindrical mirrors 448 and 449 are used tore-converge each of the two separated beams toward the seconddiffraction grating. In some such embodiments, a differentpiezo-electric actuator is attached to each of the two mirrors 448 and449 (i.e., rather than using the single piezo-electric actuator 327 onmirror 431) that are used to re-converge each of the two separated beamstoward the second diffraction grating 413, wherein the twopiezo-electric actuators allow independent adjustment of the lengths ofthe cavity 393 as seen by each intermediate beam, for tuning purposes.In other embodiments, single piezo-electric actuator 327 on mirror 431is used in conjunction with an additional single piezo-electric actuatorone or the other of mirrors 448 or 449, in order to independently tunethe lengths of the cavity seen by the two intermediate-frequency beams.In these manners, in some embodiments, the cavity 393 can beindependently frequency tuned (using the etalons 438-439 and/ordiffraction grating 412 used in conjunction with a mask having two slits(one for each of the two intermediate-frequency beams) and length tuned(using the two piezo-electric actuators) to each of the two intermediatefrequency beams, if desired.

In contrast to the device 301 embodiment shown in FIG. 3A, device 401 ofFIG. 4A separates OPO crystal 325 from DFG crystal 335 so that each isin a separate respective leg of the bow-tie ring 393 (i.e., bow-tie ring393 being the optical path from mirror 324, through OPO 325 to mirror346, then through DFG 335 and mirror 353 to diffraction grating 412 andback (via slightly separate paths for the I_(S) and I_(I) beams) tomirror 431, then the I_(S) beam goes through etalon 439 to mirror 449and then to grating 413, while the I_(I) beam goes through etalon 438 tomirror 448 and then to grating 413, where the I_(S) and I_(I) beams arecombined into a single beam, and finally back to mirror 324), whichallows removal of the pump light I_(P) through wavelength-selectivemirror 346 before the OPO-idler light I_(I) and OPO-signal light I_(S)enter the DFG crystal 335 to be converted to electromagnetic radiationof the THz output-signal frequency. This bow-tie ring path 393, whichseparates the I_(S) and I_(I) beams from one another by diffraction,allows each of the intermediate-frequency I_(S) and I_(I) beams to befiltered by an etalon specifically tuned for the two respectivefrequencies.

In other embodiments (not shown), a device that is the same as device401 but omitting etalon 438 and mirror 448 is used, in order that thering 393 is resonant only to the frequency of the I_(S) beam (the I_(I)beam being dumped), and only circulates the I_(S) beam completely aroundthe optical path of ring 393. In yet other embodiments (not shown), adevice that is the same as device 401 but omitting etalon 439 and mirror449 is used, in order that the ring 393 is resonant only to thefrequency of the I_(I) beam (the I_(S) beam being dumped), and onlycirculates the I_(I) beam completely around the optical path of ring393. In these cases, the diffraction gratings 412 and 413 and the oneetalon allow improved tuning and stability of the frequencies used togenerate the THz output signal 350. In some embodiments, some of theoptical elements that define the optical path of ring 393 are slightlyout of the plane and tilted such that the optical elements do notinterfere with the optical path 393.

FIG. 4B is a block diagram of another four-mirror bow-tie combined OPOand DFG cavity device 402. The topology and operation of device 402 aresubstantially the same as for device 401 described above. In otherembodiments (not shown), a device that is the same as device 402 butomitting etalon 438 and mirror 448 is used, in order that the ring 394is resonant only to the frequency of the I_(S) beam, and only circulatesthe I_(S) beam completely around the optical path of ring 394. In yetother embodiments (not shown), a device that is the same as device 402but omitting etalon 439 and mirror 449 is used, in order that the ring394 is resonant only to the frequency of the I_(I) beam, and onlycirculates the I_(I) beam completely around the optical path of ring394. In these cases, the diffraction gratings and etalon allow improvedtuning of the frequencies used to generate the THz output signal 350. Insome embodiments of device 402 (in contrast to some embodiment of device401 described above), all of the optical elements that define theoptical path of ring 394 are in a single plane and need not be tilted orout of the plane in order to not interfere with the optical path 394. Insome embodiments, this causes the angles between segments of the opticalpath of ring 394 to be larger (less acute) than is the case for FIG. 5Adescribed below.

FIG. 5A is an elevation-view block diagram of another four-mirrorbow-tie combined OPO and DFG cavity device 501 having a bow-tie-ringoptical path 395. The topology and operation of device 501 aresubstantially the same as for device 401 described above, except thatdevice 501 uses a single etalon 537 (optionally having a mask 536 thathas one opening for the I_(I) beam and one opening for the I_(S) beam)in place of the two etalons 438 and 439 of device 401, and device 501uses a single cylindrical mirror 545 (optionally having a mask not shownhere) in place of the two cylindrical mirrors 448 and 449 of device 401.In other embodiments, etalon 537 is omitted and mask 536, that has oneslit opening for the I_(I) beam and one slit opening for the I_(S) beam,is used to select the two intermediate frequencies (in some suchembodiments, mask 536 is configured to have one fixed slit for thefixed-frequency intermediate beam and one movable or adjustable-positionslit for the adjustable-frequency intermediate beam). In contrast tobeam-splitting diffractive grating 412 of FIG. 4A (which diffracts theI_(S) beam on one side of the combined input beam that passes throughoutput mirror 353 and diffracts the I_(I) beam on the opposite side),beam-splitting diffractive grating 334 is configured to diffract theI_(S) beam and the I_(I) beam both along respective angled paths thatare both above the combined input beam that passes through output mirror353. Also in contrast to beam-combining diffractive grating 413 of FIG.4A (which diffracts the I_(S) beam from one side of the combined outputbeam that propagates toward mirror 324, and diffracts the I_(I) beamfrom the opposite side), beam-combining diffractive grating 339 isconfigured to diffract the I_(S) beam and the I_(I) beam from theirrespective angled paths that are both above the combined output beamthat propagates toward mirror 324.

The centers of the angle-separated I_(S) beam and the I_(I) beam eachimpinge on the single etalon 537 at slightly different angles, whichallows the single etalon 537 to be used to simultaneously select (i.e.,provide a narrow-linewidth optical filter function) for the twodifferent frequencies desired for the I_(S) beam and the I_(I) beam(since the different angles of incidence provide a different spacingbetween the two faces of the etalon). Similarly, the two etalon-filteredbeams can both be focussed by a single cylindrical mirror 545 (which isconfigured to retro-reflect each of the two beams) to a single spot onthe recombining grating 339 in order to be spectral-beam combined into asingle beam directed toward M1 mirror 324. In some embodiments, theangles of divergence of the I_(S) beam and the I_(I) beam as they leavebeam-splitting diffractive grating 334 match the respective angles ofconvergence of the I_(S) beam and the I_(I) beam as they impinge towardbeam-combining diffractive grating 339.

In other embodiments (not shown), one or the other of OPO-signal beamI_(S) and OPO-idler beam I_(I) are blocked, masked, or dumped such thatonly a single intermediate frequency (i.e., either OPO-signal beam I_(S)or OPO-idler beam I_(I)) circulates completely around ring 395.

FIG. 5B is a plan-view block diagram of the four-mirror bow-tie combinedOPO and DFG cavity device 501 shown in FIG. 5A. This is simply a viewfrom a different angle of the device 501 shown in FIG. 5A.

In the drawings herein, a dashed-line arrow is sometimes used toindicate the normal vector relative to the center of the mirror face.

In some embodiments, the present invention is mounted and sealed in aunitary housing having at least one removable cover and a plurality ofoptical ports for launching pump light I_(P) into the OPO-DFG andremoving I_(THz), I_(S) and I_(I) light that result from the non-linearOPO conversion and difference frequency generation. In some embodiments,the housing is similar to that shown and described in co-owned U.S. Pat.No. 7,620,077, which is incorporated herein by reference.

In some embodiments, a set of up to four different-frequency outputbeams (i.e., I_(P), I_(S), I_(I) and/or I_(THz)) exit from the device301, 302, 401, 402 and/or 501, each beam of which can be used by itself,or in combination with other beams of this set, for various purposessuch as spectroscopy, LIDAR, LADAR, materials engineering (such as heattreating and the like), chemical processing, imaging (such as airportsecurity searches for hidden weapons, or locating firefighters or thelike, who would have THz reflectors or resonators on their person thatwould be imaged once the THz output beam hit such a device, throughsmoke or in the dark), non-lethal or lethal weapons, surgicalcoagulation, cutting, and the like.

In some embodiments, the OPO and DFG are made into one crystal withdifferent sections having different functions.

Novel attributes of some embodiments of the present invention include:(a.) high power, (b.) wide available spectral range, (c.)narrow-linewidth, (d.) widely tunable, (e.) compact, (f.) light-weight,and/or (g.) maintenance-free since they are fiber-laser based.

One purpose of this invention is to create a high-power, tunable, andnarrow-linewidth millimeter-wave or terahertz-frequency systems that canuse converted millimeter or terahertz waves for one set of functions,such as imaging, spectroscopy, non-lethal weapon and the like, whileusing its fundamental wavelength(s) that were not converted as outputfor another set of functions, such as coagulation, cutting, lethalweapons, and the like.

In some embodiments, the present invention uses two narrow-linewidthOPO-generated laser seed signals (called the OPO-signal and theOPO-idler, these are sometimes simply referred to as “seeds”), at leastone of which is tunable in some embodiments, wherein in someembodiments, the OPO pump source uses a DFB seed laser that in turn ispumped by one or more semiconductor lasers. In some embodiments, thesebeams are operated in pulsed modes. In some embodiments, the seedsignals are amplified in fiber amplifiers. Pulses from the two amplifiedseed signals are combined and sent through nonlineardifference-frequency-generation (DFG) optics. Electronics and/oralgorithms and beam-shaping optics are used to synchronize and overlapprecisely two pulses from the two fiber amplifiers' output through theDFG optics. Tunability of the seed lasers is achieved through driveconditions such as current or temperature, or acoustic optics and thelike, which are applying to key laser-cavity elements such as grating orfeedback mirrors. In some embodiments, an external enhancement cavity isused to improve DFG efficiency. In some embodiments, thulium-doped fiberamplifiers are used to amplify the two 2-μm (two-micron wavelength)signal lights for both power scalability and DEC (direct evaporativecooling) efficiency improvement. Also non-converted ˜2-μm output powercan be used for a number of other applications. Beam-shaping and-directing optics are employed to manipulate different output beams fordifferent sets of applications. In some embodiments, DFG-conversionoptics are controlled so that millimeter-wavelength terahertz outputpower is controlled.

In contrast to the present invention, electromagnetic signals inconventionally available commercial terahertz sources are typicallygenerated through either a free-electron laser or a waveguide filledwith gaseous organic molecules and generated through high-voltagedischarge through the gaseous organic molecules. These sources areneither narrow-linewidth nor widely tunable. Further, they typically arenot available in high-frequency ranges, and they are usually very largein size. A quantum cascade laser (QCL) has limited output power and canonly produce certain frequencies and requires cryogenic cooling.Terahertz generation through femtosecond lasers are usually low powerand not tunable. So, in summary, the problem posed by the combination ofrequirements has not been solved before.

In some embodiments, the present invention provides an apparatus forgenerating a gigahertz-terahertz-range signal having a frequency in agigahertz to terahertz frequency range. This apparatus includes a pumplaser that outputs pump light having a pump frequency; and a singlecavity, operably coupled to the pump laser to receive the pump light,the single cavity having non-linear material in an optical path in thecavity that receives the pump light and generates light of twointermediate frequencies, and that uses the light of the twointermediate frequencies to generate the gigahertz-terahertz-rangesignal, wherein the gigahertz-terahertz-range signal has a frequencythat is equal to a difference between the two intermediate frequencies.

In some embodiments, the optical path has a bow-tie ring topology, andwherein the single cavity further includes: a first frequency-selectivemirror that is highly transparent to the frequency of the pump light andthrough which unconverted pump light is removed from the cavity, asecond mirror that is highly reflective to at least a fixed frequency ofthe two intermediate frequencies, such that between 1% and 5% of theother of the two intermediate frequencies is transmitted through thesecond mirror, a third mirror that is highly reflective to the fixedfrequency of the two intermediate frequencies, a frequency-selectiveFabry-Perot etalon that is located in the optical path between thesecond mirror and the third mirror and that is configured to pass thefixed frequency of the two intermediate frequencies, a fourthfrequency-selective mirror that is highly reflective to the fixedfrequency of the two intermediate frequencies and highly transparent tothe frequency of the pump light and through which the pump light isintroduced into the cavity. In some such embodiments, the non-linearmaterial in the single cavity includes: a first non-linear optical (NLO)crystal of that acts as an optical parametric oscillator, and a secondnon-linear optical (NLO) crystal that acts as a difference frequencygenerator, wherein the first NLO crystal and the second NLO crystal arelocated in the optical path between the fourth mirror and the firstmirror.

In some embodiments, the optical path has a linear non-ring topology,and the non-linear material in the single cavity includes a singlenon-linear optical (NLO) crystal of that acts both as an opticalparametric oscillator and as a difference frequency generator.

In some embodiments, the optical path has a ring topology, and thenon-linear material in the single cavity includes a single non-linearoptical (NLO) crystal of that acts both as an optical parametricoscillator and as a difference frequency generator.

In some embodiments, the optical path has a ring topology, and thenon-linear material in the single cavity includes: a first non-linearoptical (NLO) crystal of that acts as an optical parametric oscillator,a second non-linear optical (NLO) crystal that acts as a differencefrequency generator, and a first mirror located in the optical pathbetween the first NLO crystal and the second NLO crystal, wherein thefirst mirror is highly reflective at both of the two intermediatefrequencies.

In some embodiments, the optical path has a ring topology, and whereinthe non-linear material in the single cavity includes: a firstnon-linear optical (NLO) crystal located in the optical path that actsas an optical parametric oscillator, a second non-linear optical (NLO)crystal located in the optical path that acts as a difference frequencygenerator, and a first mirror located in the optical path between thefirst NLO crystal and the second NLO crystal, wherein the first mirroris highly reflective at both of the two intermediate frequencies andhighly transmissive at the pump frequency.

In some embodiments, the optical path has a bow-tie ring topology, andthe non-linear material in the single cavity includes: a firstnon-linear optical (NLO) crystal of that acts as an optical parametricoscillator, a second non-linear optical (NLO) crystal that acts as adifference frequency generator and that has a first face that receiveslight of the two intermediate frequencies and a second face that emitsthe gigahertz-terahertz-range signal, a first mirror located in theoptical path between the first NLO crystal and the second NLO crystal,wherein the first mirror is highly reflective at both of the twointermediate frequencies and highly transmissive at the pump frequency,and wherein light of the two intermediate frequencies reflected by thefirst mirror enters the first face of the second NLO crystal, and asecond reflector located in the optical path facing the second face ofthe second NLO crystal, wherein the second reflector is highlyreflective at the gigahertz-terahertz-frequency range, such that thegigahertz-terahertz-range signal is reflected by the second reflectorand exits the cavity.

In some embodiments, the optical path in the single cavity is configuredto have a bow-tie ring topology.

In some embodiments, the optical path has a bow-tie ring topology, andthe apparatus further includes a unitary block housing surrounding thesingle cavity, wherein the optical path is completely within thehousing, and wherein the housing has at least one cover and a pluralityof optical ports that are coupled to the optical path.

In some embodiments, the housing also holds the pump laser in thehousing.

In some embodiments, the pump laser is configured to controllably varythe pump frequency, and the cavity is tuned to resonate at a fixed oneof the two intermediate frequencies, such that the other of the twointermediate frequencies varies based on the varied pump frequency, andsuch that the frequency of the terahertz-range signal is controllablyvaried based on the varied pump frequency.

In some embodiments, the single cavity is arranged in a ring topologyand further includes: a first frequency-selective mirror that is highlytransparent to the frequency of the pump light and through whichunconverted pump light is removed from the cavity, and highlytransparent to the frequency of the gigahertz-terahertz-range signal andthrough which the gigahertz-terahertz-range signal is removed from thecavity, a second mirror that is highly reflective to at least a fixedfrequency of the two intermediate frequencies, such that between 1% and5% of the other of the two intermediate frequencies is transmittedthrough the second mirror, a third mirror that is highly reflective tothe fixed frequency of the two intermediate frequencies, afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, and that is configuredto pass the fixed frequency of the two intermediate frequencies, afourth frequency-selective mirror that is highly reflective to at leastthe fixed frequency of the two intermediate frequencies and highlytransparent to the frequency of the pump light and through which thepump light is introduced into the cavity. In some such embodiments, thenon-linear material in the single cavity includes a single non-linearoptical (NLO) crystal of that acts both as an optical parametricoscillator, and as a difference frequency generator, wherein the singleNLO crystal is located in the optical path between the fourth mirror andthe first mirror.

In some embodiments, the single cavity further includes: a firstfrequency-selective mirror that is highly transparent to the frequencyof the pump light and through which unconverted pump light is removedfrom the cavity, a second mirror that is highly reflective to at least afixed frequency of the two intermediate frequencies, such that between1% and 5% of the other of the two intermediate frequencies istransmitted through the second mirror, a third mirror that is highlyreflective to the fixed frequency of the two intermediate frequencies, afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, and that is configuredto pass the fixed frequency of the two intermediate frequencies, afourth frequency-selective mirror that is highly reflective to at leastthe fixed frequency of the two intermediate frequencies and highlytransparent to the frequency of the pump light and through which thepump light is introduced into the cavity. In some such embodiments, thenon-linear material in the single cavity includes: a first non-linearoptical (NLO) crystal of that acts as an optical parametric oscillatorlocated in the optical path between the fourth mirror and the firstmirror, and a second non-linear optical (NLO) crystal that acts as adifference frequency generator, wherein the first NLO crystal and thesecond NLO crystal are located in the optical path between the fourthmirror and the first mirror, and a frequency-selective reflector locatedin the optical path between the second NLO crystal and the third mirror,wherein the frequency-selective reflector is configured to pass the twointermediate frequencies, and to reflect the gigahertz-terahertz-rangesignal out of the cavity.

In some embodiments, the optical path has a ring topology, and whereinthe non-linear material in the single cavity includes material that actsas an optical parametric oscillator and material that acts as adifference frequency generator.

In some embodiments, the present invention provides a method forgenerating a gigahertz-terahertz-range signal having a frequency in agigahertz to terahertz frequency range. This method includes: receivingpump light having a pump frequency into a single optical cavity;generating light of two intermediate frequencies within the singlecavity by using energy from the pump light, and generating thegigahertz-terahertz-range signal within the single cavity by using thelight of the two intermediate frequencies, wherein thegigahertz-terahertz-range signal has a frequency that is equal to adifference between the two intermediate frequencies.

In some embodiments of the method, the optical cavity has an opticalpath that has a bow-tie ring topology, and the method further includes:reflecting light of at least one of the two intermediate frequencies ata first frequency-selective mirror, and removing unconverted pump lightfrom the cavity through the first frequency-selective mirror, reflectinglight of a fixed frequency of the two intermediate frequencies at asecond mirror, reflecting light of the fixed frequency of the twointermediate frequencies at a third mirror, passing light of the fixedfrequency of the two intermediate frequencies through afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, reflecting light of thefixed frequency of the two intermediate frequencies at a fourthfrequency-selective mirror introducing the pump light through fourthfrequency-selective mirror into the cavity, converting pump light intolight of the two intermediate frequencies using non-linear opticalparametric oscillation in the optical path between the fourth mirror andthe first mirror, and converting light of the two intermediatefrequencies to electromagnetic radiation having a gigahertz-terahertzfrequency using non-linear difference frequency generation, in theoptical path between the fourth mirror and the first mirror.

In some embodiments of the method, the optical cavity has an opticalpath that has a bow-tie ring topology, and the method further includes:reflecting light of at least one of the two intermediate frequencies ata first frequency-selective mirror, and removing unconverted pump lightfrom the cavity through the first frequency-selective mirror, reflectinglight of a fixed frequency of the two intermediate frequencies at asecond mirror, reflecting light of the fixed frequency of the twointermediate frequencies at a third mirror, passing light of the fixedfrequency of the two intermediate frequencies through afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, reflecting light of thefixed frequency of the two intermediate frequencies at a fourthfrequency-selective mirror introducing the pump light through fourthfrequency-selective mirror into the cavity, converting pump light intolight of the two intermediate frequencies using non-linear opticalparametric oscillation in the optical path between the fourth mirror andthe first mirror, and converting light of the two intermediatefrequencies to electromagnetic radiation having a gigahertz-terahertzfrequency using non-linear difference frequency generation, in theoptical path between the first mirror and the second mirror.

In some embodiments, the present invention provides an apparatus forgenerating a gigahertz-terahertz-range signal having a frequency in agigahertz to terahertz frequency range. This apparatus includes meansfor receiving pump light having a pump frequency into a single opticalcavity; and means within the single cavity for generating light of twointermediate frequencies by using energy from the pump light, and forgenerating the gigahertz-terahertz-range signal by using the light ofthe two intermediate frequencies, wherein the gigahertz-terahertz-rangesignal has a frequency that is equal to a difference between the twointermediate frequencies.

In some embodiments, the optical cavity has an optical path that has abow-tie ring topology, and the apparatus further includes: means forreflecting light of at least one of the two intermediate frequencies ata first frequency-selective mirror, and removing unconverted pump lightfrom the cavity through the first frequency-selective mirror, means forreflecting light of a fixed frequency of the two intermediatefrequencies at a second mirror, and transmitting between 1% and 5% ofthe other of the two intermediate frequencies through the second mirror,means for reflecting light of the fixed frequency of the twointermediate frequencies at a third mirror, means for passing light ofthe fixed frequency of the two intermediate frequencies through afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, means for reflectinglight of the fixed frequency of the two intermediate frequencies at afourth frequency-selective mirror introducing the pump light throughfourth frequency-selective mirror into the cavity, means for convertingpump light into light of the two intermediate frequencies usingnon-linear optical parametric oscillation in the optical path betweenthe fourth mirror and the first mirror, and means for converting lightof the two intermediate frequencies to electromagnetic radiation havinga gigahertz-terahertz frequency using non-linear difference frequencygeneration, in the optical path between the fourth mirror and the firstmirror.

In some embodiments, the optical cavity has an optical path that has abow-tie ring topology, and the apparatus further includes: means forreflecting light of at least one of the two intermediate frequencies ata first frequency-selective mirror, and removing unconverted pump lightfrom the cavity through the first frequency-selective mirror, means forreflecting light of a fixed frequency of the two intermediatefrequencies at a second mirror, means for reflecting light of the fixedfrequency of the two intermediate frequencies at a third mirror, meansfor passing light of the fixed frequency of the two intermediatefrequencies through a frequency-selective Fabry-Perot etalon located inthe optical path between the second mirror and the third mirror, meansfor reflecting light of the fixed frequency of the two intermediatefrequencies at a fourth frequency-selective mirror introducing the pumplight through fourth frequency-selective mirror into the cavity, meansfor converting pump light into light of the two intermediate frequenciesusing non-linear optical parametric oscillation in the optical pathbetween the fourth mirror and the first mirror, and means for convertinglight of the two intermediate frequencies to electromagnetic radiationhaving a gigahertz-terahertz frequency using non-linear differencefrequency generation, in the optical path between the first mirror andthe second mirror.

It is specifically contemplated that the present invention includesembodiments having combinations and subcombinations of the variousembodiments and features that are individually described herein (i.e.,rather than listing every combinatorial of the elements, thisspecification includes descriptions of representative embodiments andcontemplates embodiments that include some of the features from oneembodiment combined with some of the features of another embodiment).Further, some embodiments include fewer than all the componentsdescribed as part of any one of the embodiments described herein.

All publications patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting. Some embodiments of the present invention can be used aslaboratory equipment.

As used herein the term “about” refers to ±10% inclusive. As used hereinthe term “most” refers to more than 50%.

The word “exemplary” is used herein to mean “serving as an example,instance or illustration.” Any embodiment described as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments and/or to exclude the incorporation of features from otherembodiments.

The term “in some embodiments” and the word “optionally” are used hereinto mean “is provided in some embodiments and not provided in otherembodiments.” Any particular embodiment of the invention may include aplurality of “optional” features unless such features conflict.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralversions unless the context clearly dictates otherwise. For example, theterm “a compound” or “at least one compound” may include a plurality ofcompounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It is to be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible sub-ranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6, etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6, as well asfractions for those cases not requiring an integer number. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integer) within the indicated range.The phrases “ranging/ranges between” a first indicated number and asecond indicated number and “ranging/ranges from” a first indicatednumber “to” a second indicated number are used herein interchangeablyand are meant to include the first and second indicated numbers and allthe fractional and integer numbers there between.

As used herein the term “method” refers to processes, manners, means,techniques and procedures for accomplishing a given task including thosemanners, means, techniques and procedures either known to, or readilydeveloped from known processes, manners, means, techniques andprocedures by practitioners of the optical, electrical, semiconductor,mechanical, chemical, pharmacological, biological, biochemical andmedical arts.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Although numerous characteristics andadvantages of various embodiments as described herein have been setforth in the foregoing description, together with details of thestructure and function of various embodiments, many other embodimentsand changes to details will be apparent to those of skill in the artupon reviewing the above description. The scope of the invention shouldbe, therefore, determined with reference to the appended claims, alongwith the full scope of equivalents to which such claims are entitled. Inthe appended claims, the terms “including” and “in which” are used asthe plain-English equivalents of the respective terms “comprising” and“wherein,” respectively. Moreover, the terms “first,” “second,” and“third,” etc., are used merely as labels, and are not intended to imposenumerical requirements on their objects.

What is claimed is:
 1. An apparatus for generating agigahertz-terahertz-range signal having a frequency in a gigahertz toterahertz frequency range, the apparatus comprising: a pump laser thatoutputs pump light having a pump frequency; and a single cavity,operably coupled to the pump laser to receive the pump light, the singlecavity having non-linear material in an optical path in the cavity thatreceives the pump light and generates light of two intermediatefrequencies, and that uses the light of the two intermediate frequenciesto generate the gigahertz-terahertz-range signal, wherein thegigahertz-terahertz-range signal has a frequency that is equal to adifference between the two intermediate frequencies.
 2. The apparatus ofclaim 1, wherein the optical path has a bow-tie ring topology, andwherein the single cavity further includes: a first frequency-selectivemirror that is highly transparent to the frequency of the pump light andthrough which unconverted pump light is removed from the cavity, asecond mirror that is highly reflective to at least a fixed frequency ofthe two intermediate frequencies, such that between 1% and 5% of theother of the two intermediate frequencies is transmitted through thesecond mirror, a third mirror that is highly reflective to the fixedfrequency of the two intermediate frequencies, a frequency-selectiveFabry-Perot etalon located in the optical path between the second mirrorand the third mirror, and that is configured to pass the fixed frequencyof the two intermediate frequencies, a fourth frequency-selective mirrorthat is highly reflective to the fixed frequency of the two intermediatefrequencies and highly transparent to the frequency of the pump lightand through which the pump light is introduced into the cavity, whereinthe non-linear material in the single cavity includes: a firstnon-linear optical (NLO) crystal of that acts as an optical parametricoscillator, and a second non-linear optical (NLO) crystal that acts as adifference frequency generator, wherein the first NLO crystal and thesecond NLO crystal are located in the optical path between the fourthmirror and the first mirror.
 3. The apparatus of claim 1, wherein theoptical path has a linear non-ring topology, and wherein the non-linearmaterial in the single cavity includes: a single non-linear optical(NLO) crystal of that acts both as an optical parametric oscillator andas a difference frequency generator.
 4. The apparatus of claim 1,wherein the optical path has a ring topology, and wherein the non-linearmaterial in the single cavity includes: a single non-linear optical(NLO) crystal of that acts both as an optical parametric oscillator andas a difference frequency generator.
 5. The apparatus of claim 1,wherein the optical path has a ring topology, and wherein the non-linearmaterial in the single cavity includes: a first non-linear optical (NLO)crystal of that acts as an optical parametric oscillator, a secondnon-linear optical (NLO) crystal that acts as a difference frequencygenerator, and a first mirror located in the optical path between thefirst NLO crystal and the second NLO crystal, wherein the first mirroris highly reflective at both of the two intermediate frequencies.
 6. Theapparatus of claim 1, wherein the optical path has a ring topology, andwherein the non-linear material in the single cavity includes: a firstnon-linear optical (NLO) crystal located in the optical path that actsas an optical parametric oscillator, a second non-linear optical (NLO)crystal located in the optical path that acts as a difference frequencygenerator, and a first mirror located in the optical path between thefirst NLO crystal and the second NLO crystal, wherein the first mirroris highly reflective at both of the two intermediate frequencies andhighly transmissive at the pump frequency.
 7. The apparatus of claim 1,wherein the optical path has a bow-tie ring topology, and wherein thenon-linear material in the single cavity includes: a first non-linearoptical (NLO) crystal of that acts as an optical parametric oscillator,a second non-linear optical (NLO) crystal that acts as a differencefrequency generator and that has a first face that receives light of thetwo intermediate frequencies and a second face that emits thegigahertz-terahertz-range signal, a first mirror located in the opticalpath between the first NLO crystal and the second NLO crystal, whereinthe first mirror is highly reflective at both of the two intermediatefrequencies and highly transmissive at the pump frequency, and whereinlight of the two intermediate frequencies reflected by the first mirrorenters the first face of the second NLO crystal, and a second reflectorlocated in the optical path facing the second face of the second NLOcrystal, wherein the second reflector is highly reflective at thegigahertz-terahertz-frequency range, such that thegigahertz-terahertz-range signal is reflected by the second reflectorand exits the cavity.
 8. The apparatus of claim 1, wherein the opticalpath in the single cavity is configured to have a bow-tie ring topology.9. The apparatus of claim 1, wherein the optical path has a bow-tie ringtopology, the apparatus further comprising: a unitary block housingsurrounding the single cavity, wherein the optical path is completelywithin the housing, and wherein the housing has at least one cover and aplurality of optical ports that are coupled to the optical path.
 10. Theapparatus of claim 9, wherein the housing also holds the pump laser inthe housing.
 11. The apparatus of claim 1, wherein the pump laser isconfigured to controllably vary the pump frequency, wherein the cavityis tuned to resonate at a fixed one of the two intermediate frequencies,such that the other of the two intermediate frequencies varies based onthe varied pump frequency, and such that the frequency of theterahertz-range signal is controllably varied based on the varied pumpfrequency.
 12. The apparatus of claim 1, wherein the single cavity isarranged in a ring topology and further includes: a firstfrequency-selective mirror that is highly transparent to the frequencyof the pump light and through which unconverted pump light is removedfrom the cavity, and highly transparent to the frequency of thegigahertz-terahertz-range signal and through which thegigahertz-terahertz-range signal is removed from the cavity, a secondmirror that is highly reflective to at least a fixed frequency of thetwo intermediate frequencies, such that between 1% and 5% of the otherof the two intermediate frequencies is transmitted through the secondmirror, a third mirror that is highly reflective to the fixed frequencyof the two intermediate frequencies, a frequency-selective Fabry-Perotetalon located in the optical path between the second mirror and thethird mirror, and that is configured to pass the fixed frequency of thetwo intermediate frequencies, a fourth frequency-selective mirror thatis highly reflective to at least the fixed frequency of the twointermediate frequencies and highly transparent to the frequency of thepump light and through which the pump light is introduced into thecavity, wherein the non-linear material in the single cavity includes: asingle non-linear optical (NLO) crystal of that acts both as an opticalparametric oscillator, and as a difference frequency generator, whereinthe single NLO crystal is located in the optical path between the fourthmirror and the first mirror.
 13. The apparatus of claim 1, wherein thesingle cavity further includes: a first frequency-selective mirror thatis highly transparent to the frequency of the pump light and throughwhich unconverted pump light is removed from the cavity, a second mirrorthat is highly reflective to at least a fixed frequency of the twointermediate frequencies, such that between 1% and 5% of the other ofthe two intermediate frequencies is transmitted through the secondmirror, a third mirror that is highly reflective to the fixed frequencyof the two intermediate frequencies, a frequency-selective Fabry-Perotetalon located in the optical path between the second mirror and thethird mirror, and that is configured to pass the fixed frequency of thetwo intermediate frequencies, a fourth frequency-selective mirror thatis highly reflective to at least the fixed frequency of the twointermediate frequencies and highly transparent to the frequency of thepump light and through which the pump light is introduced into thecavity, wherein the non-linear material in the single cavity includes: afirst non-linear optical (NLO) crystal of that acts as an opticalparametric oscillator located in the optical path between the fourthmirror and the first mirror, and a second non-linear optical (NLO)crystal that acts as a difference frequency generator, wherein the firstNLO crystal and the second NLO crystal are located in the optical pathbetween the fourth mirror and the first mirror, and afrequency-selective reflector located in the optical path between thesecond NLO crystal and the third mirror, wherein the frequency-selectivereflector is configured to pass the two intermediate frequencies, and toreflect the gigahertz-terahertz-range signal out of the cavity.
 14. Theapparatus of claim 1, wherein the optical path has a ring topology, andwherein the non-linear material in the single cavity includes materialthat acts as an optical parametric oscillator and material that acts asa difference frequency generator.
 15. A method for generating agigahertz-terahertz-range signal having a frequency in a gigahertz toterahertz frequency range, the method comprising: receiving pump lighthaving a pump frequency into a single optical cavity; generating lightof two intermediate frequencies within the single cavity by using energyfrom the pump light, and generating the gigahertz-terahertz-range signalwithin the single cavity by using the light of the two intermediatefrequencies, wherein the gigahertz-terahertz-range signal has afrequency that is equal to a difference between the two intermediatefrequencies.
 16. The method of claim 15, wherein the optical cavity hasan optical path that has a bow-tie ring topology, and wherein the methodfurther includes: reflecting light of at least one of the twointermediate frequencies at a first frequency-selective mirror, andremoving unconverted pump light from the cavity through the firstfrequency-selective mirror, reflecting light of a fixed frequency of thetwo intermediate frequencies at a second mirror, reflecting light of thefixed frequency of the two intermediate frequencies at a third mirror,passing light of the fixed frequency of the two intermediate frequenciesthrough a frequency-selective Fabry-Perot etalon located in the opticalpath between the second mirror and the third mirror, reflecting light ofthe fixed frequency of the two intermediate frequencies at a fourthfrequency-selective mirror introducing the pump light through fourthfrequency-selective mirror into the cavity, converting pump light intolight of the two intermediate frequencies using non-linear opticalparametric oscillation in the optical path between the fourth mirror andthe first mirror, and converting light of the two intermediatefrequencies to electromagnetic radiation having a gigahertz-terahertzfrequency using non-linear difference frequency generation, in theoptical path between the fourth mirror and the first mirror.
 17. Themethod of claim 15, wherein the optical cavity has an optical path thathas a bow-tie ring topology, and wherein the method further includes:reflecting light of at least one of the two intermediate frequencies ata first frequency-selective mirror, and removing unconverted pump lightfrom the cavity through the first frequency-selective mirror, reflectinglight of a fixed frequency of the two intermediate frequencies at asecond mirror, reflecting light of the fixed frequency of the twointermediate frequencies at a third mirror, passing light of the fixedfrequency of the two intermediate frequencies through afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, reflecting light of thefixed frequency of the two intermediate frequencies at a fourthfrequency-selective mirror introducing the pump light through fourthfrequency-selective mirror into the cavity, converting pump light intolight of the two intermediate frequencies using non-linear opticalparametric oscillation in the optical path between the fourth mirror andthe first mirror, and converting light of the two intermediatefrequencies to electromagnetic radiation having a gigahertz-terahertzfrequency using non-linear difference frequency generation, in theoptical path between the first mirror and the second mirror.
 18. Anapparatus for generating a gigahertz-terahertz-range signal having afrequency in a gigahertz to terahertz frequency range, the apparatuscomprising: means for receiving pump light having a pump frequency intoa single optical cavity; and means within the single cavity forgenerating light of two intermediate frequencies by using energy fromthe pump light, and for generating the gigahertz-terahertz-range signalby using the light of the two intermediate frequencies, wherein thegigahertz-terahertz-range signal has a frequency that is equal to adifference between the two intermediate frequencies.
 19. The apparatusof claim 18, wherein the optical cavity has an optical path that has abow-tie ring topology, and wherein the apparatus further includes: meansfor reflecting light of at least one of the two intermediate frequenciesat a first frequency-selective mirror, and removing unconverted pumplight from the cavity through the first frequency-selective mirror,means for reflecting light of a fixed frequency of the two intermediatefrequencies at a second mirror, and transmitting between 1% and 5% ofthe other of the two intermediate frequencies through the second mirror,means for reflecting light of the fixed frequency of the twointermediate frequencies at a third mirror, means for passing light ofthe fixed frequency of the two intermediate frequencies through afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, means for reflectinglight of the fixed frequency of the two intermediate frequencies at afourth frequency-selective mirror introducing the pump light throughfourth frequency-selective mirror into the cavity, means for convertingpump light into light of the two intermediate frequencies usingnon-linear optical parametric oscillation in the optical path betweenthe fourth mirror and the first mirror, and means for converting lightof the two intermediate frequencies to electromagnetic radiation havinga gigahertz-terahertz frequency using non-linear difference frequencygeneration, in the optical path between the fourth mirror and the firstmirror.
 20. The apparatus of claim 18, wherein the optical cavity has anoptical path that has a bow-tie ring topology, and wherein the apparatusfurther includes: means for reflecting light of at least one of the twointermediate frequencies at a first frequency-selective mirror, andremoving unconverted pump light from the cavity through the firstfrequency-selective mirror, means for reflecting light of a fixedfrequency of the two intermediate frequencies at a second mirror, meansfor reflecting light of the fixed frequency of the two intermediatefrequencies at a third mirror, means for passing light of the fixedfrequency of the two intermediate frequencies through afrequency-selective Fabry-Perot etalon located in the optical pathbetween the second mirror and the third mirror, means for reflectinglight of the fixed frequency of the two intermediate frequencies at afourth frequency-selective mirror introducing the pump light throughfourth frequency-selective mirror into the cavity, means for convertingpump light into light of the two intermediate frequencies usingnon-linear optical parametric oscillation in the optical path betweenthe fourth mirror and the first mirror, and means for converting lightof the two intermediate frequencies to electromagnetic radiation havinga gigahertz-terahertz frequency using non-linear difference frequencygeneration, in the optical path between the first mirror and the secondmirror.